1. Field of the Invention
This invention relates to a constant voltage generator producing a precisely controlled value of voltage, and more particularly to a reference voltage generator applicable to circuits such as an analog-digital converter or a digital-analog converter formed in a semiconductor integrated circuit.
2. Description of the Prior Art
The analog-digital converter and a digital-analog converter use a reference voltage to convert signal form. Those analog-digital converters and digital-analog converters are nowadays manufactured in a form of semiconductor integrated circuit and the required reference voltage generators are also constructed in the semiconductor integrated circuit. Generally, the output reference voltage is determined by resistance of resistor in the reference voltage generator. The resistance of resistor formed in the semiconductor integrated circuit, however, has a large deviation due to uncertainty of manufacturing process. To compensate such deviation, the resistors used in the reference voltage generator is made so that the resistances may be adjusted. The typical example of such adjustable resistor has a series connection of a main resistor and a plural number of small resistors. The total resistance is controlled by the number of small resistors. Excessive small resistors are short-circuitted.
One example of such adjustable resistors in the prior art uses polycrystalline fuses as a means for shortcircuitting. The respective polycrystalline fuses are connected in parallel with corresponding small resistors to shortcircuit them. While the total resistance of the series resistors or the output reference voltage is measured, selected number of fuses are burn out by applying a high voltage. Another example uses wiring metals in place of the polycrystalline fuses. The wiring metals are melted away by exposing to LASER light.
According to those prior arts, melted polycrystalline silicon or wiring metals is spread out to damage the wirings and elements in the semiconductor integrated circuit. Further, when the fuses or wiring metals are melted, a high melting temperature causes a thermal stress to the neighboring circuit elements. The extreme case is a deterioration of electrical characteristics of circuit elements. Furthermore, in a case where the melting of fuses or wiring metals is insufficient, the separated parts of fuses or wiring metals reconnects in the long run. Finally, the polycrystalline silicon fuses are melted by applying a very high voltage. Therefore, the high voltage affects the neighboring circuit elements to deteriorate their electrical characteristics.
As mentioned above, the use of fuses or wiring metals lowers the reliability and the production yield.